1. Technical Field
The present invention relates to a method of removing a metal skin from a through-hole surface of a copper-Invar-copper (CIC) laminate without causing differential etchback of the laminate.
2. Related Art
A copper-Invar-copper (CIC) laminate is commonly included in electronic packages. For example, a CIC laminate may be present within a dielectric substrate. Exterior surfaces of the substrate may be circuitized such that a plated through hole (PTH) passes through the CIC laminate and electrically couples the surface circuitizations. During formation of the preceding electrical structure, laser or mechanical drilling of a through hole that passes through the CIC laminate generates debris along the drilling path. A portion of the debris forms a porous "metal skin" over a CIC surface within the through hole. The predominant constituents of the metal skin are metallic debris derived from the CIC laminate, such as copper, iron, and nickel. The metal skin may also include a relatively small amount of dielectric debris derived from the dielectric substrate.
The metal skin is problematic because its porosity allows processing chemicals, such as acid solutions, to penetrate the pores of the metal skin and become trapped between the metal skin and the CIC surface covered by the metal skin. Subsequent copper plating of the through hole to form the PTH may seal the acid solutions or other chemicals between the metal skin and the CIC surface covered by the metal skin. The sealing of acid solution is a reliability concern. For example, a pocket void may be generated within the Invar layer of the CIC laminate due to formation of a galvanic cell comprising a CIC copper layer, the CIC Invar, and the acid solution. Noting that Invar is an alloy that includes iron (Fe) and nickel (Fe), the galvanic cell causes oxidiation of the Invar and consequent formation of iron and/or nickel ions. Such ions dissolve in the acid solution and are thus permanently removed from the Invar, thereby creating a pocket void in the Invar layer. Such pocket voids impair the structural integrity of the overlaying copper plating and may subject the copper plating to mechanical and thermal stresses.
Another adverse aspect of the metal skin is that the metal skin is an unnecessary interface between the copper plating and the CIC surface covered by the copper plating. This extra interface could fail, mechanically or thermally, even in the absence of pocket voids.
Fabricating an electronic device having a CIC laminate with a drilled through hole may include removing the metal skin that covers the through hole and etch-cleaning the CIC surface beneath the metal skin. Thus, a method is needed for removing the metal skin and etching the underlying CIC surface such that the depth of CIC material etched away is about the same for the copper and Invar layers of the CIC laminate.